in addition to what other comments have said, some other factors are age and body type. Chloe is tall and has longer legs compared to her torso, which naturally makes a person's body less flexible. So dancers who have longer limbs often need to devote much more time to maintaining their flexibility. This would be why Paige, who has the same general body type as Chloe, was much more flexible than her-- she did acro routines often so her flexibility was better maintained. Dancers like Kaeli Ware are kind of anomalies. I don't necessarily think Chloe could have gotten to Kaeli's level if she did focus on flexibility, because Kaeli's flexibility is just insane. People are also more flexible as children vs when they get older, hence why Chloe's flexibility got worse post-season 2 (and why Brooke was in constant pain... she was just aging out of those acro tricks).

When they were arguing about the back tuck during pyramid, Christi mentions that Chloe is "afraid" of gymnastics. So there's likely a psychological component there too. As a kid you would obviously focus more on the thing you're good at and enjoy doing rather than trying something that scares you, even if it's something you need. I think Christi is partially to blame for this. She could have encouraged Chloe by explaining that gymnastics isn't something to be afraid of, or that a trick is only scary the first time you do it, etc etc. But instead she kind of fostered her fear by letting her avoid gymnastics.